i2c: designware: fix building driver as module

The designware core and platform are built as separate modules.
Export i2c_dw_prepare_clk() so it can be used by the platform
driver.

Fixes: a34a0b6d ("i2c: designware: move i2c_dw_plat_prepare_clk to common")
Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>